[clang] [z/OS] Set the default arch for z/OS to be arch10 (PR #89854)

[Sean Perry via cfe-commits]

perry-ca wrote:

I removed the additional use of CLANG_SYSTEMZ_DEFAULT_ARCH and left that for a separate discussion.  The only code clean up left here is the elimination of the `CPU` member that wasn't used.

https://github.com/llvm/llvm-project/pull/89854